The present invention relates to a method of controlling gas-borne particulates, and more particularly, but not exclusively, to controlling the migration and deposition of contaminating airborne particulates. It further relates to apparatus for achieving such control which may be used, for example, to obtain a work space substantially devoid of airborne particulate contaminants, both biological and non-biological, such as is necessary for the manufacture of sterile pharmaceutical products and micro-electronic components. Another use is in hospitals, where isolation of patients and prevention of contamination during surgical procedures is intended.
Previously proposed airborne contamination control methods include the widely used Laminar Air Flow System for equipment like work stations and sterilizing tunnels, and also for clean or sterile rooms. These rely on airflow in which the entire body of air within a confined area essentially moves with uniform velocity along parallel flow lines. This velocity through the cross section of the work space, is normally maintained at 27.5 meters per minute with a uniformity within plus or minus 20%. When the airflow is vertically downward this velocity can be lower, but with the uniformity maintained. The intention is to stratify the air such that minimum cross-stream particulate migration occurs, with the result that activity-generated particulates that become airborne are carried linearly along a predictable path.
These previously proposed methods are deficient in that when an object, for example, the workpiece or manufacturing equipment or operator, whose presence is unavoidable in practical work situations, is introduced into the airstream, the laminar air flow is altered such that it is not possible to regulate the particulate content within the work space as would be possible without the object altering the air flow pattern.
It is known that when an object is introduced into moving streams of air, a higher pressure exists on the upstream side than on the downstream side of the object. The pressure gradient around the object creates turbulence, which in turn leads to eddy currents around the object, with the consequent back-mix effects, which entrap in the proximity of an obstructing object particles shed by the object into the air mass around the object. This reduces the cleanliness levels otherwise obtainable.
Most articles found in a work station are, in fact, resting on one or other flat, horizontal surfaces. For example, operators and equipment stand on floors, work-pieces on tables, and articles to be sterilized travel on horizontal conveyor belts. See U.S. Pat. Nos. 3,977,091 and 4,140,479. In a horizontal flow configuration, the path of a flow of gas across these articles is thereby blocked at the lower extremities of the obstruction. Air streams must either go around or over the blockade which, in uniform velocity flow, they do at accelerated velocities. Manual movements in relation to a workpiece, which usually generate contaminants, tend to be in the vertical plane. This stems from the operator's need to maintain an ergonomic focal distance between his eyes and the work activity. These vertical movements add to the effects of the low pressure zone in the wake region which also alters the flow axis of the streamlines in the separated boundary above the object so that the direction of flow is downward, thereby orienting entrained particles into deposition trajectories.
The hitherto proposed uniform velocity laminar air flow systems are also liable to be affected by particulate migration caused by different energy gradient forces. Free particulates travel to the region where the energy level is lowest. For example, particulates settle under the influence of gravity; and, where a thermal gradient exists, particulates tend to move towards the cooler zone; and statically charged particulates move towards a neutralising field. In practical work situations, a single uniform velocity does not effectively counter all these energy differentials.
To circumvent these deficiences of uniform velocity air flow systems, many methods have been proposed. These include the use of air-directing devices that cause an air flow to converge, or to focus, onto the obstructing work objects such that purging of the low pressure zone with filtered air is achieved. As these air streams intersect, the inherent advantages of predictability that parallel streamlines, not disturbed by the obstruction, have in removing particulate contaminants is compromised. See U.S. Pat. No. 3,426,512.
Likewise, for hospital operating rooms, it has been proposed to utilize air-directing vanes which deflect at acute angles the air emanating from the filter face towards the site of the surgical activity. The air flow, on encountering the downstream wall, is retro-directed along return paths separated by a zero velocity boundary, thus obviating the use of partitions. See U.S. Pat. No. 3,721,067. Another method, suggested for work benches, is to short-cycle the air flow and entrained particulates, downstream of the obstruction, through apertures located on the edges of the containment surfaces including the work table. See U.S. Pat. No. 3,378,076.
It is an object of this invention to provide a method for filtering and conveying air unidirectionally in parallel streamlines, each of which moves at a different velocity selectively predetermined and graded across the cross-section of the air path, such that air-borne particulates, which migrate due to aerodynamic drag and other kinetic forces and/or potentiated by thermal or electrostatic energy differentials, are regulated more efficaciously.
It is another object of this invention to provide a device having instrumentalities for generating predetermined air streams that are parallel; where each stream moves at a rate of flow which is different and graded such that particulate migration can be better controlled.
It is a further object of this invention to increase the number of air-borne particulates that will stay in linear orientations further along a clean to dirty axis, when such particulates are released into a forward or reverse multiple velocity gradient cross-flow clean or sterile room, or clean air work station.
It is another object of this invention to decrease the particulate contamination during sterilization and depyrogenation of objects, for example, glass containers for sterile pharmaceutical products by previously heated air, while at the same time to provide an air flow oriented to transfer heat at thermodynamically designed gradients.